The present invention generally relates to the manufacture of defluorinated food and feed grade phosphates from phosphate rock. Phosphate rock usually occurs as natural deposits of fluorapatite, a phosphate of lime containing varying amounts of fluorine, chlorine, hydroxyl, and silica and is generally represented by the formula Ca.sub.10 (PO.sub.4).sub.6 F.sub.2. Many attempts have already been made to convert raw phosphates, with the assistance of added materials such as SiO.sub.2, by fusion in the presence of steam (U.S. Pat. Nos. 2,143,865; 2,220,572; 2,895,799). The chemical reaction is as follows: EQU 2Ca.sub.5 (PO.sub.4).sub.3 F+SiO.sub.2 +H.sub.2 O.fwdarw.3Ca.sub.3 (PO.sub.4).sub.2 +CaSiO.sub.3 +2HF
The energy necessary to drive this reaction at the desired temperature of about 1930.degree. C is 1234 BTU per pound of solids produced. The required amount of steam would be: ##EQU1## AND THE RESULTANT STEAM ENTHALPY WOULD BE: 123410.0169=73,000BTU/lb. Enthalpies of this magnitude cannot be attained using existing arc heater equipment.
Presently, steam enthalpies of only up to 8,000 BTU/lb. are possible. A large excess of steam must be used, so that sufficient energy can be supplied to drive the reaction. In present processes, such large excesses of steam serve little chemical function, but merely act as a fairly inefficient heat carrier. The desired reaction temperature for rapid kinetics is 1930.degree. therefore, only the energy invested in the steam above that temperature, i.e. an enthalpy in excess of 3500 BTU/lb. can be used to heat the reactants and supply the heat of reaction.
Process efficiency for this prior art method, using arc heated steam at 8,000 BTU/lb., has been calculated at about 44%, with a total electrical cost of about $17/ton of deflourinated phosphate, at a production rate of about 1,300 lb. rock/hr. There is a need for a faster, more efficient and economical method for arc heater production of defluorinated phosphates.